1. Field of the Invention
This invention relates to a hot water boiling apparatus of a storage type, using an electric heater as its heat source.
2. Description of the Related Art
Hot water boiling apparatuses using an electric heater as its heat source are classified into the instantaneous type and the storage type. The instantaneous type is constructed such that water is heated instantaneously to a certain temperature by the use of a large-capacity electric heater to supply hot water. The storage type is constructed such that hot water at a fixed temperature is previously stored in a hot water storage tank and the hot water is supplied when necessary. With an instantaneous type boiling apparatus, a sufficient amount of hot water cannot be supplied unless an electric heater with a capacity as large as 5 to 20 kw is used. For this reason, in the general households, storage type boiling apparatus are used exclusively.
Normally, storage type hot water boiling apparatus have a hot water storage tank, the outer surface of which is covered with a heat insulating material such as glass wool. The bottom of the storage tank is connected with a water supply pipe. The top of the tank is connected through a hot water supply pipe to a tap. A sheath-type electric heater is located at the bottom of the inside of the hot water storage tank. The whole water in the storage tank is kept heated to 80.degree. C., for example, by supplying power to the electric heater and hot water is taken out through the hot water supply pipe when necessary. As for the method of producing water when a hot water boiling system for houses is composed using such a hot water supply, there are two methods: the centralized method in which a single large hot water boiling apparatus supplies to a number of places and the decentralized method in which small hot water boiling apparatuses are installed at the respective places of use. The centralized method has a problem that cold water comes out for a while due to cooling of the pipe after the tap is opened. Hence, the decentralized method is currently finding growing use.
When a conventional hot water boiling apparatus is reduced simply in size and used in a decentralized system, however, there is a problem as follows. In the decentralized method, the amount of hot water consumption at each place of use is necessarily small. It happens therefore, the heat loss due to radiation from the hot water storage tank is greater than the heat quantity of hot water consumed by actual use of hot water. To take an example, suppose a hot water storage tank 250 mm in inner diameter and 400 mm high, with a volume of 19.6 liters and a surface area of 0.412 m. Also suppose that the outer surface of the hot storage tank is covered with glass wool 25 mm thick and the head conductivity of glass wool is 0.035 kcal/m.sup.2 .degree. C. h. Then, the heat loss through the heat insulating material is as follows. If the hot water temperature in the storage tank is 85.degree. C. and the ambient temperature is 15.degree. C., the heat loss Hl (kcal/h) is 40.38 (kcal/h). That is to say, heat loss a day is 1.13 kwh. If the heat loss is calculated in terms of amount of hot water, 20 liters of 68.degree. C. hot water is wasted a day assuming that the temperature of water supplied is 20.degree. C. A possible solution to this problem is to use a hot water storage tank of the vacuum heat insulation type excellent in diabatic performance. To install an electric heater in the hot water storage tank, however, it is necessary to provide a heater insertion passageway that passes through the vacuum heat insulation space. This not only increases the production cost of hot water storage tanks but causes heat loss through the heater-inserted portion of the storage tank, thus considerably reducing the effects of use of a vacuum heat insulation type of hot water storage tank. With a hot water boiling apparatus having an electric heater installed in the hot water storage tank, when power is supplied to the electric heater under the condition that, for example, hot water of 80.degree. C. remains in the upper one third of the tank and water of 10.degree. C. is present in the lower two thirds of the tank, as the electric heater begins to heat the water, thermal convection takes place, causing the whole water in the tank to be stirred. As a result, the temperature of the whole area in the tank falls uniformly to 33.degree. C. for a time. Therefore, it is impossible to instantly supply hot water at an adequate temperature. Thus, conventional hot water boiling apparatuses have difficulty in quickly responding to the need.
As described above, if an attempt is made to use conventional hot water boiling apparatuses in a decentralized hot water supply system by reducing their size, a great heat loss can occur through the outer surfaces of the hot water storage tanks. In addition, an unfavorable phenomenon peculiar to the natural convection heating method takes place, which causes difficulty in quick response to demand.